1. Field of the Invention
This invention relates to video monitors. In particular, the invention relates to monitor control.
2. Description of Related Art
Multi-scan or point-scan monitors can accommodate a wide range of operating frequency. One important parameter in the performance of monitors is the horizontal scanning frequency which is related to the display resolution. For a selected display resolution, a multi-scan video monitor generates a proper horizontal frequency to the deflection driving circuit that drives the cathode ray tube (CRT). When a video monitor switches from one horizontal frequency to another to support several display resolutions, it should maintain the linearity of the deflection circuit.
The horizontal linearity of a video monitor is a performance measure to show the consistency and uniformity in the display. It is therefore important to maintain the horizontal linearity over the entire operational frequency range to keep picture distortion at the minimum. The horizontal linearity is controlled by the deflection control circuit in the monitor. One way to control the deflection control circuit is to use programmable capacitors and/or resistors to vary the time constants of the timing generator circuits, or to add a correction factor in the timing circuits. For a given set of frequencies, it is possible to calculate a corresponding set of capacitor values that should be used to control the deflection control circuit. These values form an S-correction capacitor table, usually referred to as an SCAP table.
During testing, diagnostic, or field use, the values of the capacitors may be changed to accommodate the particular video monitor or the capacitance drift due to aging and other environmental and operating conditions. To provide the flexibility of updating the SCAP table, a programmable non-volatile memory is used to store the SCAP table.
The updating of the SCAP table in the programmable memory has a number of problems. First, the updated data may be corrupted by noise in the programming of the non-volatile memory. Second, the updated data may be incorrectly chosen due to operator's mistakes. Third, the updated data may be erroneously read due to hardware failure in the communication link between the controller and the non-volatile memory. When incorrect updated values for the SCAP table are used, the deflection control circuit may generate wrong control signals that degrade the horizontal linearity of the video monitor.
Therefore there is a need in the technology to provide a simple and efficient method to provide proper capacitor values to the video control circuit to maintain horizontal linearity over the operating range of frequencies.